This invention relates to the electronic cardiac pacer art, and more particularly to a new and improved electronic cardiac pacer having stimulation rate control.
Early in the development of electronic cardiac pacemaking there appeared the nonsynchronous pacer which provides fixed rate stimulation, and although the stimulation is not automatically changed in accordance with the body's needs, it has proven effective in alleviating the symptoms of complete heart block. A nonsynchronous pacer, however, has the possible disadvantage of competing with the natural, physiological pacer during episodes of normal sinus conduction.
As a result, the demand-type pacer was developed having the capability that artificial stimuli are initiated only when required and subsequently can be eliminated when the heart returns to the sinus rhythm. The demand pacer solves the problem encountered with the nonsynchronous pacer by inhibiting itself in the presence of ventricular activity but coming "on line" and filling in missed heartbeats in the absence of ventricular activity.
More recently, a cardiac pacer of the demand type was developed wherein pulse generation is locked in timing relation to a source of timing signals which operates at a constant frequency. In this pacer the timing element includes digital clock circuitry of the integrated circuit type, and the pacer encourages the natural heart rate to conform to a precise repetitive signal. In particular, this digital pacer operates from a quartz crystal through a frequency divider chain to give output pulses at a rate of approximately one per second. Such a pacer has a very precise rate which will never change appreciably and will never run away. It would be highly desirable to provide such a pacer having the capability of control of stimulation rate and with such rate changes advantageously being made form an external controller.